A rare-earth alloy sintered magnet (permanent magnet) is normally produced by compacting a powder of a rare-earth alloy, sintering the resultant powder compact and then subjecting the sintered body to an aging treatment if necessary. Permanent magnets currently used extensively in various applications include rare-earth-cobalt based (typically samarium-cobalt based) magnets and rare-earth-iron-boron based (typically neodymium-iron-boron based) magnets. Among other things, the rare-earth-iron-boron based magnets (which will be referred to herein as “R—Fe—B based magnets”, where R is one of the rare-earth elements including Y, Fe is iron, and B is boron) are used more and more often in various electronic appliances. This is because an R—Fe—B based magnet exhibits a maximum energy product, which is higher than any of various other types of magnets, and yet is relatively inexpensive.
An R—Fe—B based sintered magnet includes a main phase consisting essentially of a tetragonal R2Fe14B compound (which will be sometimes referred to herein as an “R2Fe14B type crystal layer”), an R-rich phase including Nd, for example, and a B-rich phase. In the R—Fe—B based sintered magnet, a portion of Fe may be replaced with a transition metal such as Co or Ni and a portion of B may be replaced with C. An R—Fe—B based sintered magnet, to which the present invention is applicable effectively, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example, the entire contents of which are hereby incorporated by reference.
In the prior art, an R—Fe—B based alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare-earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing a solid alloy (i.e., alloy ingot). A method for obtaining a solid alloy by a Ca reduction process (which is also called a “reduction diffusion process”) is also known.
Recently, a rapid cooling process (which is also called a “melt-quenching process”) such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, a molten alloy is brought into contact with, and relatively rapidly cooled by, a single chill roller, a twin chill roller, a rotating disk or the inner surface of a rotating cylindrical casting mold, thereby making a solidified alloy, which is thinner than an alloy ingot, from the molten alloy.
A solid alloy obtained by a rapid cooling process will be referred to herein as a “rapidly cooled alloy (or rapidly solidified alloy)” so as to be easily distinguished from a solid alloy obtained by a conventional ingot casting process or Ca reduction process. The rapidly solidified alloy typically has the shape of a flake or a ribbon (thin strip).
Compared to a solid alloy made by the conventional ingot casting process or die casting process (such an alloy will be referred to herein as an “ingot alloy”), the rapidly solidified alloy has been quenched in a shorter time (i.e., at a cooling rate of 102° C./sec to 104° C./sec). Accordingly, the rapidly solidified alloy has a finer texture and a smaller crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phases are dispersed broadly and thinly over the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phases. Because the rapidly solidified alloy has these advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alloy powder to be compacted is obtained by coarsely pulverizing a rapidly solidified alloy in any of these forms by a hydrogen pulverization process, for example, and/or any of various mechanical grinding processes (e.g., using a ball mill or attritor) and finely pulverizing the resultant coarse powder (with a mean particle size of 10 μm to 500 μm) by a dry pulverization process using a jet mill, for example. The alloy powder to be compacted preferably has a mean particle size of 1 μm to 10 μm, more preferably 1.5 μm to 7 μm, to achieve sufficient magnetic properties. It should be noted that the “mean particle size” of a powder refers herein to an FSSS particle size unless otherwise stated.
A rapidly solidified alloy powder obtained in this manner is typically processed into compacts by a uniaxial compacting process. Due to its manufacturing method, the rapidly solidified alloy powder has a narrow particle size distribution and achieves a bad fill density (i.e., cannot fill the cavity to a desired fill density), which are both problems.
Thus, to improve the fill density of the rapidly solidified alloy powder, various countermeasures have been proposed. For example, Japanese Patent Application Laid-Open Publication No. 2000-219942 describes that if a rapidly solidified alloy, including 1 vol % to 30 vol % of chilled texture with particle sizes of 3 μm or less, is made by a roller rapid cooling process and then pulverized to obtain a rapidly solidified alloy powder, then the fill density can be increased and the sintering temperature can be decreased compared with conventional ones.
It should be noted that the “chilled texture” is a crystalline phase to be formed near the surface of a chill roller during an initial stage in which a melt of an R—Fe—B based rare-earth alloy has just contacted with the surface of a cooling member (e.g., the chill roller) of a rapid cooling system and started to solidify. Compared with a columnar texture (or dendrite texture) to be formed on and after that initial stage of the cooling and solidification process, the chilled texture has a more isotropic (or isometric) and finer structure. That is to say, the chilled texture is produced when the melt is rapidly cooled and solidified on the surface of the roller.